The pilot of the oil tanker that hit the Bay Bridge last week apparently decided at the last minute to change course and head between a different set of bridge towers, a tricky maneuver that may have played a role in the accident, experts said.

The course change put him more at the mercy of a current that cuts beneath the bridge, and with a thick fog that suddenly swept in as he approached the towers, pilot Guy Kleess would have had less margin for error as he steered the Overseas Reymar the morning of Jan. 7, according to the experts.

The empty, 751-foot tanker sideswiped the western span tower closest to Yerba Buena Island, causing more than $2 million in damage to the tower's wooden fenders but spilling no oil into the bay.

Coast Guard investigators are looking into a range of possible causes including human error and equipment failure. The agency has said Kleess received a warning from its Vessel Traffic Service, which monitors all big ships in the bay, before he hit the tower, but the timing and nature of that warning are still being examined.

Peter McIsaac, president of the San Francisco Bar Pilots association, said that when Kleess left his anchorage spot south of the Bay Bridge, he was heading for an opening between the nautically termed C and D towers of the Bay Bridge, which is the second opening between towers from the west side of Yerba Buena.

"But then he switched and headed for Delta-Echo," McIsaac said, referring to the opening between the D and E towers, which stand on either side of the opening closest to the island. "I don't know why he changed direction, and that sort of information will have to come out in the investigation."

The "racon" radar beacon that helps guide ships between the C and D towers was not functioning on that day, according to Caltrans, which run the units, but the racon for the D and E tower opening was working. Coast Guard investigators said it is unsure how that may have affected the accident.

Tricky currents

The 2,210-foot-wide Delta-Echo opening is the one most commonly used by commercial ships going under the Bay Bridge, and at the time of the accident the ebb current for the day was flowing near its charted maximum for the day of 4.7 knots. Even with a slow current, McIsaac said, that spot requires pilots to take extra care because it's where ebb flows from Oakland and from the South Bay meet and swirl into each other.

"Taking a ship under the bridge is not like driving on a road, because with a ship the road is actually moving," McIsaac said.

John Konrad, a ship's captain who runs the widely used gcaptain.com maritime blog, said that when changing course, a ship that size tends to slide sideways.

'Definitely unusual'

He also said that GPS-generated charts from Jan. 7 and reports from other sailors confirm the change in direction for the Overseas Reymar, and that such a change "is definitely unusual."

"If he was planning to go through that original span, he would have lined up with it much earlier," Konrad said. "If you are in heavy fog and you're unsure of the oncoming traffic, you might turn like that - and my best guess is that maybe he was trying to be careful."

Visibility was about a half-mile when the Overseas Reymar left its anchorage, but dropped to a quarter-mile in the half-hour it took to get to the bridge, McIsaac said.

Konrad said the ship was traveling at more than 11 knots, which may have been too fast. The Cosco Busan container ship was traveling at 10.6 knots when it hit the bridge in 2007 in heavy fog, spilling 53,000 gallons of oil into the bay, and the Coast Guard determined that was too fast.

"You could conclude that (Kleess) should have been going slower, but the investigators will have to determine that," Konrad said.

He said the safest tactics on the morning of the crash, if there was any uncertainty about visibility, would have been either to bring a tugboat alongside to help with directing, or to head back to anchorage.

"However, tugs are expensive and stopping at anchorage is expensive," Konrad said. "So a pilot in that situation is left with a lot of pressure."